Atomic-Scale Understanding of the Role of Dopant (Al, Zr) on the Structural Properties of Nickel-Rich Cathode for Lithium-Ion Batteries

Although Lithium-ion batteries (LIBs) have been widely used in various electronic devices due to their high specific energy densities and stable cycling performance, a quest for further improvement in structural stability and high capacity of LIBs is still on going.^[1] Recently, Sun et al., demonstrated that development of Ni-rich cathode (LiNi_xCo_yMn_1-x-yO₂, x ≥ 0.6) can drastically improve the capacity of LIBs.^[2] However, the incorporation of Ni results in structural degradation by causing cation mixing at the bulk region and irreversible phase transition at the surface which eventually deteriorates the performance of LIBs. As a countermeasure, a number of research studies have indicated that surface doping using Al and Zr can suppress structural degradation of Ni-rich cathode.^[3],[4] Nevertheless, in-depth understanding of doping mechanism for instance, exact doping site is still not fully explored because of their extremely low concentration making them difficult to analyze.<br/>This study employs synergistic combination of atomic-scale scanning transmission electron microscopy (STEM), energy dispersive spectroscopy (EDS) and density functional theory (DFT) to investigate the doping mechanism such as doping site and structural stabilization of Al- and Zr-doped Ni-rich cathode. The atomic scale study shows that minimization of structural degradation after cycle is highly achieved with Zr-, followed by Al doping compared to undoped Ni-rich cathode. In addition, atomic-scale EDS analysis reveal for the first time that the doping sites of Al and Zr were transition metal sites. DFT calculation further shows that Zr-doped Ni-rich cathode has the largest cation mixing formation energy and that Zr is the most strongly bonded to oxygen than Al and Ni. In conclusion, Zr inhibits cation mixing by blocking the movement of Ni ions, and the strong bond between Zr and O prevents oxygen release and suppresses the phase transition from the surface, thereby improving the structural stability of the Ni-rich cathode.<br/><br/><br/><br/>References<br/>[1] Dan Gao et. <i>Journal of Materials Chemistry A</i> 7.41 (2019) 23964-23972.<br/>[2] Noh, Hyung-Joo, et al. <i>Journal of power sources</i> 233 (2013) 121-130.<br/>[3] Zou, Lianfeng, et al. <i>Nature communications</i> 10.1 (2019) 1-11.<br/>[4] Schipper, Florian, et al. <i>Advanced Energy Materials</i> 8.4 (2018) 1701682.